on garge doors they have a saftey beam that goes across the front of the door created with and ir led what is?

Question

used on the reciveing end what is the part called that recieves the signal it looks just like the ir led.

Its not a passive inferared sensor(pirs) like for motion sensing lights its much less complicated tahn that its more like the sensor in your tv. I have done a small project with these before where an ir diode stays lit and then it makes a circuit with the reciever when a match box car goes past it breaks the signal and then it would record a time. Any way both the sensor and the reciever looked like leds is it posibble a led could be used to close a circuit as a reciever?

Answer 1

It depends a lot on the exact system, but if it looks just like the IR-LED then it is probably an IR Phototransistor.

A phototransistor works just like a regular transistor except that instead of using base current to "turn on" the transistor, light falling on the face of it allows current to flow.

In an application like this, the phototransistor simply allows the beam of IR to generate a "clear to close" logic signal back to the motor controller.

So now imagine your matchbox car idea, with an IR-LED on one side of the track and a phototransistor on the other side. With no car blocking the beam, IR light from the IR-LED is seen by the IR phototransistor and the phototransistor conducts electricity. If the emitter of an NPN phototransistor is grounded and the collector is connected to a 5-volt supply through about a 2.4K resistor, the collector will read somewhere near 0 volts as long as there is no car there. But when a car breaks the beam, the phototransistor stops conducting and the voltage at the collector becomes close to 5 volts. This is a pretty clear logic-level signal that you can use to do other things.

If you go to www.digikey.com and look up their part numbers QSC112-ND you will see these are good IR phototransistors. I have used the QSC112 in many projects to detect things breaking light beams. The QSC112 is not available at Radio Shack but I think they do have anothe phototransistor there.

Another device that might be used is the Cadmium Sulfide Photocell, or CdS Cell:
http://www.robotroom.com/PhotocellGrabBag.html
These act more like resistors whose resistance changes based on how much light is falling on them. They are slower to react than a phototransistor though, so they work best for slow signals. But detecting matchbox cars in a racetrack would probably be well within their ability. Radio Shack used to sell these, but I don't think they still do. I get mine from www.mpja.com or www.jameco.com -- they have a pretty good variety.

Answer 2

They are IR [Infrared] Sensors.